1. Field of the Invention
The present invention relates to a controller which controls a drive speed of a vibration-type motor.
2. Description of the Related Art
A vibration-type motor includes a vibrating body which is made of a metal elastic body or the like to which an electromechanical energy conversion element (also referred to as a piezoelectric element or an electrostrictive element) is bonded, and a contacting body which is pressed to be in contact with the vibrating body. When a plurality of frequency signals having a phase difference are applied to the piezoelectric element, a vibration (for example, a progressive vibrational wave) is excited and the vibrating body and the contacting body are relatively moved to generate a driving force.
As a speed control method of such a vibration-type motor, there is a method of changing a frequency of a frequency signal which is applied to the piezoelectric element (hereinafter, referred to as a frequency control). The relationship between the frequency and the speed is as illustrated in FIG. 11A. The maximum speed can be obtained at a resonance frequency (fr), and the speed is lowered as the frequency is away from the resonance frequency. This is because the diameter of an elliptical orbit which is depicted by a mass point on a surface of the vibrating body is increased as the frequency is closer to the resonance frequency. The characteristic of the frequency and the speed shows a stable characteristic of a moderate speed change at a high-frequency side with reference to the resonance frequency, as compared with a characteristic at a low-frequency side with reference to the resonance frequency. Therefore, for easiness of the control, the speed control is performed in a frequency range at the high-frequency side with reference to the resonance frequency in many cases. The vibration-type motor decelerates as the frequency is higher, and it stops by losing the driving force by the friction between the vibrating body and the contacting body at a predetermined frequency (fm).
As another speed control method of the vibration-type motor, there is a method of changing a phase difference of a plurality of frequency signals which are applied to the piezoelectric element (hereinafter, referred to as a phase difference control). The relationship between the phase difference and the speed is as illustrated in FIG. 11B. When the phase difference is set to 90 degrees or −90 degrees, the maximum speed can be obtained, and the speed is lowered as the phase difference is away from ±90 degrees. This is because the ratio of a vibration amplitude in a relative movement direction of the vibrating body and the contacting body and a vibration amplitude in a direction perpendicular to the relative movement direction is changed and therefore the shape of the elliptical orbit of the mass point is changed. This is different from a case in which the frequency is changed to change the diameter of the elliptical orbit to change the speed.
The characteristic indicated by a solid line in FIG. 11B is a characteristic of a case in which the frequency is set to the resonance frequency (fr), and the characteristic indicated by a dashed line is a characteristic of a case in which the frequency is set to a frequency (f0) that is higher than the resonance frequency illustrated in FIG. 11A and that is lower than the frequency fm. In both frequencies, the speed of the vibration-type motor is unstable if the phase difference is away from ±90 degrees, and it stops if the phase difference comes close to zero. The speed obtained at the frequency f0 is lower than a case in which the frequency is set to the resonance frequency (fr), and the speed is lower than the minimum speed obtained by the change of the frequency or the amplitude.
Considering the speed control characteristic of the vibration-type motor, the frequency control and the phase difference control are combined to be able to widen a controllable speed range. Japanese Patent Laid-open No. 6-237584 and Japanese Patent Laid-open No. 4-75479 disclose a method of controlling the speed of the vibration-type motor by combining the frequency control and the phase difference control.
Japanese Patent Laid-open No. 6-237584 discloses a speed control method of improving the resolution of the speed control by combining the frequency control and the phase difference control. Japanese Patent Laid-open No. 4-75479 discloses a speed control method of combining the frequency control and the phase difference control for the purpose of lowering the driving speed immediately before the stop in the phase difference control.
In the speed control method disclosed in Japanese Patent Laid-open No. 6-237584, the speed control resolution by the phase control is set to be small with respect to the speed control resolution by the frequency control. When the speed cannot come close to a target speed by the frequency control in the same speed range, the control is switched to the phase control only once to come close to the target speed. In other words, in a specific speed range, the speed control resolution is just improved by switching the control method only once, and it does not widen a controllable speed range. Accordingly, the original speed variable range in which the vibration-type motor can be driven cannot be sufficiently used.
In the speed control method disclosed in Japanese Patent Laid-open No. 4-75479, when the difference between a real driving position and a target driving position of an object to be driven by the vibration-type motor is smaller than a predetermined value, the frequency control is switched to the phase difference control only once. This is not necessarily switched to the phase control at a frequency close to the frequency fm described above, and the original speed variable range in which the vibration-type motor can be driven cannot be sufficiently used.